[Weather] Acoustic rain gauge

[Dale Chatham]

There are a number of problems with this approach, I think.

1.  Not all raindrops are the same mass.  One could not, therefore, 
assume volume = #hits * raindrop size.  One could, I suppose, measure 
the energy of each raindrop, calculate its mass from the energy.
2.  Not all raindrops fall at the same angle.  This is probably a rather 
serious limitation, as I don't see how you could normalize on angle 
without some sophistication of the instrument.  One could take the 
arccos(angle)*result of calculation in #1.
3.  Not all raindrops fall at the same velocity.  Wind blown rain is 
likely faster than wind not acted on by rain.  Larger drops probably 
fall faster than smaller drops.  (Mass is the cube of radius of a drop, 
speed goes as the cross sectional area, wich is a square function).

So, in order to successfully make such a measurement, one would, it 
seems to me, need to accurately measure (or derive from measurements) 
the following:

Raindrop mass.
Raindrop speed.
Raindrop direction.

In order to have meaningful results, one would also have to sample 
across an area sufficient to ensure that it was representative of the 
whole.  I'd think the proper area would be of the order of ten square 
inches to a square foot.

One might be able to simplify the problem somewhat by collecting rain 
and allowing it to drip from the reservoir, counting drops.  This would 
simplify the problem by ensuring equal sized drops with equal velocity 
falling vertically.  However, one would not be able to measure rainfall 
rate instantaneously, as one can do to sme degree with the tipping bucket.



Frédéric wrote:
> Le jeudi 26 avril 2007 12:55, Matthew Smith a écrit :
>
>   
>> Measurement of rainfall by this means is not - as far as I can see -
>> feasible.  Rainfall is about volume (per area); an acoustic transducer
>> could only really measure the product of the velocity of an incoming
>> drop multiplied by its mass (will the real physics guys help me out
>> here?)  These "hits" would not contain the data to measure the
>> volume/area.
>>     
>
> Tell me if I am wrong, but because of the air, and because a drop is not 
> solid, its speed only depends on its mass, no? I mean, falling from 100m or 
> 1000m, it will reach the same speed (if a physician could confirm...) So, 
> measuring the energy of the sound can give its mass, so its volume.
>
> Air temperature and pressure are parameters to take into account... ut if 
> there is wind, the drop will not produce the same sound, as it will not hit 
> the surface with the same angle.
>
> Not very easy, so :o/
>
>